The present invention relates generally to the formation of oxidation-resistant metallic coatings on metal components. More particularly, the present invention provides an interlayer which affords improved compatibility between the alloy HR-120 and aluminum-containing oxidation-resistant coatings.
HR-120 is a Nixe2x80x94Fexe2x80x94Cr alloy with good high temperature properties which permit its substitution for more expensive Ni-based superalloys in some gas turbine hot section components. HR-120 has a nominal composition (wt %) of Ni 37; Cr 25; Co 3 max; Mo 2.5 max; W 2.5 max; Cb 0.7; Mn 0.7; Si 0.6; Al 0.1; C 0.05; B 0.004; N 0.2; balance Fe. However, if the temperature of operation is high enough, oxidation-resistant coatings may be required to protect the alloy from excessive oxidation attack. The oxidation coatings used in gas turbines are typically aluminum rich, since Al is the preferred element for forming a protective oxide scale on the surface of the component.
HR-120 is a rather unique high temperature alloy in that it incorporates an intentional addition of nitrogen which serves as a solid solution strengthener. High temperature exposure of HR-120 samples coated with the sprayed MCrAlY (M is Co, Ni and/or Fe) and diffusion aluminide coatings (i.e., coatings formed when Al is diffused into the substrate and other elements in the substrate form the coating along with the aluminum), typically used on turbine buckets, nozzles and shrouds, revealed the formation of aluminum nitride phases, either at the coating/substrate interface in the case of the MCrAlY coating, or within the coating itself in the case of the diffusion aluminide coatings. In the latter case, some of the coatings were rendered totally ineffective by this interaction and simply spalled off during a static air exposure. In the MCrAlY coating case, a continuous Al nitride layer was formed along the coating/substrate interface. Being a brittle phase, such a layer is not a desirable microstructure, since delamination of the protective coating would be less difficult than if no such layer existed.
The formation of these brittle nitride phases is the consequence of the availability of N in the HR-120. The nitrogen readily combines with the Al in the coating when the two elements encounter each other as a result of the interdiffusion of the two materials at high temperatures.
A need exists to avoid the formation of brittle nitride layers during the service exposure of coated HR-120. The present invention seeks to meet that need.
It has now been discovered that the formation of a brittle Al nitride phase as a result of interaction of the N in the HR-120 with the Al in the coating can be eliminated or inhibited by depositing an interlayer of material between the HR-120 and the oxidation-resistant coating. Surprisingly, the presence of such an interlayer can be accomplished with minimum effect upon the mechanical and physical properties of the coated alloy.
In a first aspect, the present invention provides a coated metal component containing an intentional addition of nitrogen, comprising an oxidation-resistant coating layer and an intermediate layer disposed between the oxidation-resistant coating layer and the component. The intermediate layer is substantially devoid of nitrogen which if present would form a brittle nitride phase with the aluminum from the oxidation-resistant layer.
In a further aspect, the present invention provides a method of forming a coating on a metal component containing an intentional addition of nitrogen, which comprises coating a first intermediate layer onto the component, with the intermediate layer being substantially devoid of nitrogen. An oxidation-resistant layer is then coated onto the intermediate layer.